Trying to get the heater working

Ok... are you sure the fan is running the right direction? The original had one terminal so it was impossible to wire it backwards but since that one has two, then it's possible. Even if the HVAC has no vacuum, it will default to the defroster and heater normally.

Next, in the photo of the tan part of the selector:

There are only two hoses. You are missing one, the rear one. On my car, the front port(toward the trunk) has a green hose. When I say green I mean black with green stripe and that will apply from here on.

The rear port has a white vacuum hose. The green one leads out to the water valve, the white leads around to connect to the other vacuum lines.

Speaking of which-

There is supposed to be way more than just two vacuum lines connected to the selector. There is also a vacuum "harness" that has a molded end that fits onto another part of the selector. Take a look at the selector in this link and you will see what I mean in the 4th picture.

[ame="http://www.ebay.com/itm/1973-1974-1975-1976-DART-DUSTER-A-AC-HEATER-CONTROL-ASSEMBLY-PLYMOUTH-DODGE-/271463542929?pt=Vintage_Car_Truck_Parts_Accessories&hash=item3f347cf091&vxp=mtr"]1973 1974 1975 1976 Dart Duster A AC Heater Control Assembly Plymouth Dodge | eBay[/ame]

The vacuum dashpot that is broken off the side of the box is the defroster actuator.

My guess is the PO did all that damage when removing the heater box to replace the core. You should go back and ask him to refund some money for the aggravation. The Factory Service manual shows exactly how to remove the heater core without pulling the entire box.

I have the vacuum diagram for the heat/air if you need me to scan it.